Liquid Gas Vaporization and Measurement System and Method

ABSTRACT

A liquid gas vaporization and measurement system, and associated method, for efficiently vaporizing a continuous sample of liquid gas, such as liquid natural gas (LNG), and accurately determining the constituent components of the gas. A constant flow of liquid gas sampled from a mass storage device is maintained in a vaporizing device. Within the vaporizing device the liquid gas is flash vaporized within heated narrow tubing. The liquid gas is converted to vapor very quickly as it enters one or more independently operating vaporizer stages within the vaporizing device. The vapor gas is provided to a measuring instrument such as a chromatograph and the individual constituent components and the BTU value of the gas are determined to an accuracy of within +/−0.5 mole percent and 1 BTU, respectively.

I. CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Ser. No.11/358,724 filed on Feb. 22, 2006, now U.S. Pat. No. 7,484,404, andclaims priority of Provisional Application Ser. No. 60/654,472, filed onFeb. 22, 2005, the entire contents of both the patent and theprovisional application are incorporated herein by reference.

II. FIELD OF THE INVENTION

This invention relates generally to a system and method for theefficient vaporization and measurement of liquid natural gas (LNG). Moreparticularly, the invention relates to a system and method forcontinuously and efficiently vaporizing an LNG shipment, or portionthereof, into its gaseous form in order to accurately determine theconstituent components and British Thermal Unit (BTU) value of the LNGshipment.

III. BACKGROUND OF THE INVENTION

Natural gas is a combustible, gaseous mixture of several differenthydrocarbon compounds and is typically extracted from deep undergroundreservoirs formed by porous rock. The composition of natural gasextracted from different reservoirs varies depending on the geographiclocation of the reservoir. In fact, it is not entirely uncommon for thecomposition of gas extracted from a single given reservoir to vary to anextent. Regardless of any variations, however, the primary component ofnatural gas is methane, a colorless, odorless, gaseous saturatedhydrocarbon. Methane usually accounts for 80% to 95% of any natural gassample and the balance is composed of varying amounts of ethane,propane, butane, pentane and other hydrocarbon compounds.

Natural gas is used extensively in residential, commercial andindustrial applications. It is the dominant energy used for home heatingwith well over half of American homes using natural gas. The use ofnatural gas is also rapidly increasing in electric power generation andcooling, and as a transportation fuel.

Natural gas, like other forms of heat energy, is measured in Britishthermal units or Btu. One Btu is equivalent to the heat needed to raisethe temperature of one pound of water by one degree Fahrenheit atatmospheric pressure.

A cubic foot of natural gas has about 1,027 BTU. Natural gas is normallysold from the wellhead, i.e., the point at which the gas is extractedfrom the earth, to purchasers in standard volume measurements ofthousands of cubic feet (Mcf). However, consumer bills are usuallymeasured in heat content or therms. One therm is a unit of heating equalto 100,000 BTU.

Three separate and often independent segments of the natural gasindustry are involved in delivering natural gas from the wellhead to theconsumer. Production companies explore, drill and extract natural gasfrom the ground; transmission companies operate the pipelines thatconnect the gas fields to major consuming areas; and distributioncompanies are the local utilities that deliver natural gas to thecustomer.

In the United States alone, natural gas is delivered to close to 200million consumers through a network of underground pipes that extendsover a million miles. To produce and deliver this natural gas there areover a quarter-million producing natural gas wells, over one hundrednatural gas pipeline companies and more than a thousand localdistribution companies (LDCs) that provide gas service to all 50 states.

Prior to regulatory reform, which essentially restructured the industry,producers sold gas to the pipeline companies, who sold it to the LDCs,who sold it to residential and other customers. Post-regulation,however, pipeline companies no longer purchase gas for resale. Instead,the pipeline companies merely transport gas from sellers, such asproducers or marketers, to buyers, such as electric utilities, factoriesand LDCs. Thus, the LDCs now can choose among a variety of sellers ofnatural gas, whereas before they could only buy gas from one source,i.e., the pipeline company. Further, some states have implementedadditional restructuring which renders the LDCs subject to regulation byState public utility commissions. Prior to these additional stateregulations, an LDC's residential customers could only buy gas from onesource, i.e., the LDC. After state regulation, however, residentialcustomers can choose a different supplier other than their LDC fromwhich to buy the gas. The consumer's LDC, as the owner/operator of thedistribution network, delivers the gas to the consumer, as before, butthe LDC only charges the consumer for delivery of the gas and theindependent supplier bills for the gas.

Thus, natural gas is very important to the U.S. energy supply.Consumption of natural gas in the United States, however, has increasedbeyond the available supply of domestic natural gas. One availableoption to increase supply is to increase imports of liquefied naturalgas (LNG).

More particularly, according to one estimate natural gas consumption inthe United States is expected to increase from about 22 trillion cubicfeet (Tcf) in 2004 to almost 31 Tcf by 2025. Accordingly, domesticproduction combined with imports via pipeline from Canada will beinsufficient to meet the demand. In response, a small but growingpercentage of gas supplies are imported and received as LNG via tankerships.

LNG is produced by taking natural gas from a production field, removingimpurities, and liquefying the natural gas. In the liquefaction process,the gas is cooled to a temperature of approximately −260 degrees F. Onevolume of this condensed liquid form of natural gas occupies about1/600th of the volume of natural gas at a stove burner tip. The LNG isloaded onto double-hulled ships which are used for both safety andinsulating purposes. Once the ship arrives at the receiving port, theLNG is typically off-loaded into well-insulated storage tanks.Vaporization or regasification is used to convert the LNG back into itsgas form, which enters the domestic pipeline distribution system and isultimately delivered to the end-user.

Because LNG is sold in accordance with its BTU value, accurate analysisof the BTU value of any particular LNG shipment, as well as analysis ofthe constituent components of the LNG, as it is off-loaded from arespective tanker ship is crucial. For example, to determine an expectedprice for a particular shipment, when LNG is loaded onto a tanker shipat an overseas location, such as Trinidad and Tobago where large naturalgas reserves are found, the supplier calculates the Btu value of the LNGas it is loaded into the hull of the ship. Additionally, because the Btuvalue of the shipment will likely change in transit, for example due tovaporization of some of the LNG while it is sitting in the hull of theship, the recipient of the LNG shipment also desires to accuratelydetermine the Btu value of the delivered LNG shipment. The operator ofthe tanker ship carrying the LNG shipment is also keenly interested inaccurate BTU measurement of both the loaded LNG as well as theoff-loaded LNG as the shipper typically burns the LNG vaporized intransit to run the ship and, thus, is responsible for cost of the LNGvaporized in transit.

Accordingly, it is desired to provide a method and system for accuratelymeasuring the BTU value of an LNG shipment as it is off-loaded from atanker ship.

One related art method that addresses the issue discussed above isdisclosed in U.S. Pat. No. 3,933,030 to Forster et al. In Forster, asystem is disclosed for the continuous monitoring of the density ofcryogenic liquids, such as LNG. In accordance with the Forster systemthe dielectric constant of stored LNG is instantaneously determined bythe use of sensors in the storage tank. Multiple sensors, eachcomprising a capacitor probe, are placed at various locations within thestorage tank. The sensors are then operable to instantaneously measurethe dielectric constant of the liquid within the tank and from this datathe density of the liquid in the tank is determined. From the densitymeasurement it is possible to then calculate the BTU per unit volume andappropriate charges per BTU can be calculated.

Several problems arise from a system such as the one disclosed inForster, however. For example, the accuracy of the BTU measurement isunacceptable for today's standards.

Other, more recent, related art systems utilize chromatograph technologyto determine the BTU value of LNG. These related art systems, however,also suffer from poor accuracy and/or high levels of maintenance. Forexample, one known system utilizes a method in which liquid gas iscirculated in tubes that are submersed in a heated solution. The heat inthe solution, in turn, heats the tubing which vaporizes the liquid gas.This method of vaporization is very inefficient, however, and theaccuracy of any resulting BTU measurements are unacceptable, e.g., lessthan 5 BTU, that is, the swing on the BTU measurement is greater than 5BTU.

Accordingly, it is desired to provide a system that does not suffer fromat least these problems and which can provide a much more accurate anddetailed assessment of liquefied gas and at the same time requires lessmaintenance than current systems.

IV. SUMMARY OF THE INVENTION

Illustrative, non-limiting embodiments of the present invention mayovercome the aforementioned and other disadvantages associated withrelated art liquid gas vaporization and measurement systems. Also, thepresent invention is not necessarily required to overcome thedisadvantages described above and an illustrative non-limitingembodiment of the present invention may not overcome any of the problemsdescribed above.

It is an object of the present invention to provide a novel system andmethod for efficiently and accurately sampling and measuring liquid gas.

To achieve the above and other objects an embodiment in accordance withthe invention includes a system for vaporizing and measuring liquid gas,the system comprising a transmission device operable to transmit liquidgas, a measurement device operable to continuously extract at least aportion of the liquid gas from the transmission device while it is beingtransmitted by the transmission device, convert the extracted liquid gasfrom liquid form to vapor form and determine the constituent componentsof the vapor gas.

Another embodiment of the invention includes a device for sampling andvaporizing liquid gas, the device comprising a first stage vaporizeroperable to receive liquid gas at a first flow rate from an input portand convert the received liquid gas into vapor gas, a second stagevaporizer operable to receive liquid gas at a second flow rate andconvert the received liquid gas into vapor gas, an accumulator connectedto the first and second stage vaporizers and operable to receive andstore the vapor gas, and a third stage vaporizer connected to theaccumulator and operable to receive stored vapor gas from theaccumulator and control the pressure of the received vapor gas to bewithin a desired pressure range. In this exemplary embodiment stainlesssteel tubing is used to convey the gas throughout the system.Additionally, the tubing within the second stage vaporizer has adiameter as small as one-eight inch and is spirally wound around one ormore cartridge heaters to efficiently flash vaporize the liquid gas. Aconstant flow of gas through the system is also maintained by using aspeed loop.

An even further embodiment of the invention includes a method ofmeasuring the constituent components of liquid gas, the methodcomprising receiving the liquid gas into a vaporizing device,selectively directing the received liquid gas into one or more of afirst and second stage vaporizer within the vaporizing device,converting the liquid gas directed to one or more of the first andsecond stage vaporizers into vapor gas and accumulating the vapor gas ina relatively small storage device, for example, one-half cubic footvolume. The exemplary method further includes outputting the vapor gasaccumulated in the storage device and directing the outputted vapor gasfrom the storage device to a measuring device operable to determine theconstituent components of the vapor gas.

As used herein “gas” means any type of gaseous matter capable of pipetransmission, including natural gas, organic gases, industrial gases,medical gases, monomolecular gases, gas mixtures, and equivalents.

As used herein “connected” includes physical, whether direct orindirect, permanently affixed or adjustably mounted. Thus, unlessspecified, “connected” is intended to embrace any operationallyfunctional connection.

As used herein “substantially,” “generally,” and other words of degreeare relative modifiers intended to indicate permissible variation fromthe characteristic so modified. It is not intended to be limited to theabsolute value or characteristic which it modifies but rather possessingmore of the physical or functional characteristic than its opposite, andpreferably, approaching or approximating such a physical or functionalcharacteristic.

In the following description, reference is made to the accompanyingdrawings which are provided for illustration purposes as representativeof specific exemplary embodiments in which the invention may bepracticed. The following illustrated embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. It is to be understood that other embodiments may be utilizedand that structural changes based on presently known structural and/orfunctional equivalents may be made without departing from the scope ofthe invention.

Given the following detailed description, it should become apparent tothe person having ordinary skill in the art that the invention hereinprovides a novel liquid gas vaporization and measurement system and amethod thereof for providing significantly augmented efficiencies whilemitigating problems of the prior art.

V. BRIEF DESCRIPTION OF THE DRAWINGS

The aspects of the present invention will become more readily apparentby describing in detail illustrative, non-limiting embodiments thereofwith reference to the accompanying drawings, in which:

FIG. 1 is block diagram illustrating a system in accordance with thepresent invention.

FIG. 2 is a schematic diagram of a vaporizing and measurement device inaccordance with the present invention.

FIG. 3 is a drawing of an embodiment of a second stage vaporizeraccording to the invention.

FIG. 4 is a drawing of an alternative embodiment of according to theinvention.

VI. DETAILED DESCRIPTION OF ILLUSTRATIVE, NON-LIMITING EMBODIMENTS

Exemplary, non-limiting, embodiments of the present invention arediscussed in detail below. While specific configurations and dimensionsare discussed to provide a clear understanding, it should be understoodthat the disclosed dimensions and configurations are provided forillustration purposes only. A person skilled in the relevant art willrecognize that, unless otherwise specified, other dimensions andconfigurations may be used without departing from the spirit and scopeof the invention.

FIG. 1 is an exemplary block diagram illustrating a system in accordancewith the present invention. As shown, the system of FIG. 1 comprises atanker ship 1 carrying a shipment of liquefied natural gas (LNG). Inaccordance with this embodiment, tanker 1 docks into a port where theshipment of LNG is to be off-loaded to storage tanks before beingregassified and shipped to various gas customers. According to thepresent embodiment tanker ship 1 is a marine vessel. However, a skilledartisan would understand that tanker ship 1 could also be any vehicle ordevice capable of transporting or storing liquefied gas, such as atanker truck or other storage device. To accurately measure the BTUvalue of the LNG being offloaded from tanker 1, a vaporizer unit 2 inaccordance with the present invention and discussed in detail below, isconnected in-line with the LNG being transferred from tanker 1 tostorage tank 4 via, for example, pipeline 3. As LNG is transported inpipeline 3 to storage tank 4, at least a portion of the LNG is deliveredto vaporizer unit 2 to be analyzed and measured.

As discussed in detail below, vaporizer unit 2 continuously receives anamount of LNG from pipeline 3, vaporizes the LNG into gaseous form andanalyzes the vaporized LNG to very accurately determine the constituentcomponents of the gas, for example, via a chromatograph. Thus, on acontinuous basis, that is, continually as the LNG is being transportedin pipe 3 to storage tank 4, the real-time, or at least verynear-real-time, BTU value for the LNG being transported is calculated.Accordingly, an accurate accounting of the LNG and its BTU value and/orcost is determined for the LNG being offloaded or otherwise transferredinto storage tank 4. It should be noted that not only is the placementof the vaporizer unit 2 important for such calculations, e.g., the LNGvaporizer unit 2 should be as close to the LNG discharge line aspossible, but also the structure and configuration of the vaporizer unitadditionally contributes to extremely accurate calculations of the BTUvalue of the LNG.

The LNG from which the representative sample is extracted and used inunit 2 is pumped or otherwise transferred into storage tank 4 where itis kept at the appropriate pressure and temperature to reduce both therisk of explosion as well the risk of inadvertent vaporization into theatmosphere. The LNG resides in tank 4 until it is needed, e.g., in theform of natural gas vapor for consumers, upon which time the LNG ispumped from tank 4 and regassified, or vaporized, by degasificationdevice 5. Degasification or vaporization device 5 can be any one orcombination of known vaporization devices. For example, vaporizationdevice 5 can be an open rack vaporizer (ORV), a submerged combustionvaporizer (SCV), a combined heat and power unit with SCV (CHP-SCV), anambient air-heated vaporizer or any combination of these or other typesof vaporizers.

After the bulk-stored LNG for consumption by consumers has beenconverted into vapor gas, the vapor gas is transferred, for example, viaa pipeline system 6, to local distributors, i.e., the LCDs, and to theend-users. At any point after the LNG has been turned back into gas byvaporization device 5, the gas can be, but in accordance with theinvention does not have to be, sampled and conditioned via a Gas SampleConditioning System 7 such as the one disclosed in U.S. patentapplication Ser. No. 11/169,619, which assigned to the same assignee asthe present invention.

FIG. 2 is a schematic diagram of an LNG vaporizer unit 2 in accordancewith the present invention. In accordance with the exemplary embodimentof vaporizer unit 2 shown in FIG. 2, LNG is input to cabinet 10 via LNGinlet port 11 which comprises, for example, stainless steel tubinghaving a diameter of ¼-inch. Cabinet 10 comprises an enclosure forproviding protection from the elements, such as rain, snow, ice, wind,etc. to the individual components within. Inlet port 11 is connected to¼ inch tubing within enclosure 10 which, in turn connects to first andsecond vaporizer stages 12, 13, respectively. The first and secondvaporizer stages 12 and 13 operate independently to vaporize LNG intoits gaseous form. In particular, LNG enters inlet port 11 in liquid format a temperature of approximately −249° F., although a person ofordinary skill in the art would understand that LNG remains in liquidform at temperatures generally below 100° F. and, thus, consistent withthe invention other temperatures are possible as well. The LNG input toinlet port 11 is then selectively channeled to one or both of the firstand second stage vaporizing devices 12, 13.

Because LNG begins to vaporize as soon as it begins to heat up and thelonger a tube carrying LNG is, the warmer the LNG gets, the tubescarrying the LNG within enclosure 10 and connecting the various deviceswithin the vaporizer unit 2 are kept as short as possible, i.e., tominimize the amount of vaporization that takes place prior to the LNGentering one or both of the first and second stage vaporizing devices12, 13. Also, insulation, such as two inches of polyisocyanateinsulating material, is placed on and around the ¼ inch tubing thatcarries the LNG from the input port to each of the first and secondstage vaporizer devices.

Valve 14 is attached to ¼ inch tubing that connects the inlet port 11 tofirst stage vaporizer 12. Valve 14 operates to shut-off or open the pathfor LNG to flow into the first stage 12. The first stage vaporizer 12uses a heated spiraled entry (not shown) as well as exiting heattransfer and the gas output exits at approximately 100° F. at a flowrate of 18 SCFH (standard cubic feet per hour).

As gas exits the first stage vaporizer 12 it travels through ¼ inchtubing to the top of accumulator device 18. Accumulator device 18 is agas cylinder capable of storing natural gas vapor.

The second stage vaporizer 13 is connected to the inlet port 11 viaadditional ¼ inch tubing and one or more valves 15, 16. The second stagevaporizer 13 comprises a plurality cartridge heaters 13 a, 13 b, 13 caround each of which is wound a length of ⅛ inch tubing. For example, asshown in FIG. 2, three cartridge heaters each have respective lengths of⅛ inch tubing wound around their outer surface. The tubing around eachof the heaters is connected to the ¼ inch tubing carrying the LNG to thestage 2 vaporizer.

It should be noted that valves 14-17, ideally, are suitable forcryogenic operation due to the low temperatures of the LNG flowingtherethrough. Accordingly, valves 14-17 are optional and not necessarilyrequired for the operation of the LNG cabinet.

FIG. 3 is a close-up view of an exemplary second stage vaporizer similarto second stage vaporizer 13 shown in FIG. 2. The second stage vaporizershown in FIG. 3 utilizes four cartridge heaters 113 a through 113 d, asopposed to the three cartridge heaters shown in regard to the embodimentof FIG. 2. Otherwise, the second stage vaporizer shown in FIG. 3 isidentical to the one depicted in FIG. 2. Also, as shown in FIG. 3, arespective length of ⅛ inch tubing t1-t4 is wound around each cartridgeheater 113 a-113 d.

Referring to FIG. 3, the LNG enters the second stage vaporizer at thebottom via four respective ¼ inch input tubes IT₁-IT₄. Within the secondstage vaporizer the LNG is then directed into four respective ⅛ inchtubes, t1-t4. Each tube t1-t4 is wound spirally around a respectivecartridge heater 113 a-113 d that quickly heats the LNG within thenarrow spiral tubing converting the LNG within the tubes to vapor gas.The vapor gas from each of the respective ⅛ inch tubes is then directedinto a respective output tube OT₁-OT₄ and the vapor gas is directed intothe accumulator in similar fashion as discussed with respect to FIG. 2.

Referring back to FIG. 2, respective valves (not shown) control the flowof LNG into the respective tubing wound around each of the cartridges 13a, 13 b and 13 c. In particular, the flow into each of the ⅛ inch tubesis controlled such that the total flow of LNG in the ¼ inch tube flowsthrough the three ⅛ inch tubes in any combination, e.g., ⅓ in each ofthe three ⅛ inch tubes, ½ in each of two ⅛ inch tubes and none in thethird ⅛ inch tube, etc. Further, it should be noted that valves 14-17are also configured such that the LNG that enters the input port 11 canbe directed in any desired percentage to both the first and second stagevaporizers 12 and 13. For example, it is possible to direct any amount,X, (where X=0% to 100%) of the LNG entering port 11 into the first orsecond stage vaporizer and an amount Y (where Y=100−X) into the other ofthe first and second stage vaporizers. Accordingly, it is possible torun the vaporizer cabinet 10 even if one of the first or second stagevaporizers should fail.

It should also be noted that even though the present embodiment includesthree cartridge heaters, e.g., 13 a, 13 b and 13 c, the invention is notlimited to this configuration. One of ordinary skill would know thatprovided sufficient LNG/vapor flow through the second stage vaporizer,any number of cartridge heaters can be used.

As vapor gas exits the second stage vaporizer 13 the vapor gas iscarried by ¼ inch tubing to accumulator 18. As shown, the vapor gasenters accumulator 18 at the top and is carried via a tube 19 inside theaccumulator to an interior location within the tank 18. As vapor gasexits the tube 19 it is directed toward the inside wall of the tank 18.As the vapor gas impinges the interior wall of tank 18 it is mixedthoroughly with any gas already existing within the tank. Tube 19 is ofvariable length and can expel vapor gas within tank 18 at any heightwithin the tank. However, in accordance with the present embodiment, theoutput of tube 19 is approximately 80 to 90 percent down toward thebottom of the tank.

Thoroughly mixed vapor gas within accumulator tank 18 is removed viaadditional tubing 20 near the top of tank 18. The removed gas is carriedin ¼ inch tubing 21 to a “T” joint 22. At “T” 22 the vapor gas is eitherdirected into tubing 28, through valve 23 or some combination of both.Valve 23 controls the amount of vapor gas permitted to flow intovaporizer stage 3 (ref. no. 24). Vaporizer stage 3 essentially operatesas a pressure reducer. That is, stage 3 (24) controls the pressure forvapor permitted to enter tube 26, which carries the sample vapor gas toa chromatograph, discussed later. For example, in accordance with onescenario, vaporizer cabinet 10 is positioned in close proximity to apipeline header carrying LNG from a tanker ship to on or more storagetanks (See, e.g., FIG. 1). As the storage tank 4 begins to fill with LNGthe pressure within pipe 3 must increase in order to continue to filltank 4. As the pressure in pipe 3 increases, so does the pressure in thesample line to cabinet 10 and through the various stages ofvaporization. Accordingly, stage 3 (24) operates to control the pressureof vapor gas into tube 26. For instance, the pressure in tube 21 andthrough valve 23 might be somewhere around 10-65 PSI. Pressures in thisrange are typically detrimental to chromatograph devices and, thus,stage 3 reduces the pressure to an acceptable level, such as 5-10 PSIG.Vapor gas at an acceptable pressure is then output from cabinet 10 atport 29.

According to the embodiment shown in FIG. 2, a tube 27 is connected to a“T” joint 25 which is further connected to tube 26. Tube 27 is furtherconnected to a relief valve 30 which releases vapor gas therethrough inthe event the pressure in tube 26 should exceed a predetermined maximumvalue. That is, relief valve 30 normally does not permit gas to flowthrough it when the pressure in tube 26 is below a certain value. If,however, the pressure in tube 26 exceeds this value, relief valve 30opens and releases an amount of gas necessary to reduce the pressure intube 26 to below the predetermined value. Any vapor gas released byrelief valve 30 goes through one-way valve 31 and is provided to an LNGvapor return line via tube 32.

Any vapor gas outputted from accumulator 18 that does not pass throughvalve 23 and into stage 3 (24) enters tube 28 and exits cabinet 10 atport 33. One or more valves, V_(n), are provided to control gas flowinginto sample tanks ST_(n). For example, one or more sample tanks (e.g.,ST1-ST5) are provided to store samples of vapor gas withdrawn fromaccumulator 18. For instance, different samples can be taken and storedat different times, such as at various times during the overallunloading process of a load of LNG from a tanker ship as it istransferred into a storage tank. Valves V_(n) are individually opened orclosed in order to store samples in sample tanks ST_(n) at appropriatetimes.

The gas stored in any one of the sample tanks STn can be controlled tocome directly from the output of accumulator 18 or it can be a sampletaken from the output of vaporizer stage 3 (24). For example, duringperiods when a tanker ship is not being off-loaded, the LNG beinginputted to input port 11 is recirculated LNG from a storage tank, suchas tank 4 shown in FIG. 1. By recirculating LNG from storage tank 4 inthis manner, a constant pressure (and temperature) is maintained in thelines of vaporizer 10. Because the pressure in the main line 3 is notsignificantly altered, as compared to the situation when a tanker isbeing off-loaded as described above, it is not necessary to regulate, orreduce, the pressure using stage 3 (24).

Thus, under these circumstances sample LNG is vaporized by one or moreof stages 1 and 2 (12 and 13 in FIG. 2), vapor is collected and mixed inaccumulator 18 and the vapor is drawn off through tubes 21 and 28 andout cabinet 10 through port 33. The vapor from the recirculated LNGsample is then directed to either sample tanks STn, bypassed aroundsample tanks STn and returned to the LNG vapor return line or channeledthrough one of the valves Vn and into chromatograph 52 via optionalliquid block 51. For example, optional liquid block 51 is used fornatural gas production gas where liquids are typically present.Similarly, if desired, by opening or closing the appropriate combinationof valves Vn, the vapor gas outputted from stage 3 (24) is directed tothe sample tanks STn, bypassed around sample tanks STn and returned tothe LNG vapor return line or channeled through one of the valves Vn andinto chromatograph 52 via optional liquid block 51.

In order to calibrate chromatograph 52, a tank of calibration gas with aknown composition is stored in cal tank 50. Accordingly, when it isdesired to calibrate the chromatograph 52, the vapor gas outputted fromcabinet 10, through either port 29 or port 33, is shut-off automaticallyand calibration gas from tank 50 is applied to the chromatograph 52.

While various aspects of the present invention have been particularlyshown and described with reference to the exemplary, non-limiting,embodiments above, it will be understood by those skilled in the artthat various additional aspects and embodiments may be contemplatedwithout departing from the spirit and scope of the present invention.

For example, FIG. 4 illustrates and alternative embodiment of avaporizer cabinet which differs somewhat from vaporizer 2 shown in FIG.2. The vaporizer shown in FIG. 4 is similar in most respects to thevaporizer shown in FIG. 2. However, the embodiment of FIG. 4 uses afour-cartridge heater similar to the one illustrated in FIG. 3 andvarious other components are configured differently.

In particular, as shown in FIG. 4, LNG is input to the vaporizer cabinet110 through inlet port 111 located near the top of the cabinet 110. Afirst stage vaporizer 112 receives a portion of the LNG and a secondstage 113 receives the balance of the LNG. It should be noted that thepipe lengths for the pipes bringing LNG into the cabinet from the headerpipe 3 (FIG. 1) are kept as short as possible to minimize any heating ofthe LNG within the inlet pipes. The second stage vaporizer 113 utilizesfour cartridge heaters as shown in FIG. 3. Both the first and secondstages heat the LNG and convert it to vapor gas which is accumulated inaccumulator 118. Also, connected to each of the first and second stages,112 and 113, as well as the accumulator 118, is tubing 132 that exhaustsfrom the cabinet 110 at outlet 133 to an LNG vapor return line (notshown). Heater 135 is located within the LNG vaporizer cabinet to keepthe outlet tubes at or above a minimum temperature, for example, suchthat the gas within the outlet tubes remains in gaseous form.

Additionally, with respect to the embodiment shown in FIG. 4, the systempressure is monitored, as opposed to monitoring the pressure into thechromatograph, as is the case in regard to the embodiment of FIG. 1. Inparticular, vapor pressure is sampled at approximately the middle ofaccumulator tank 118. The vapor is removed through port 134 and thepressure is measured. Accordingly, by monitoring the system pressuredata is provided with respect to the unloading pump sequences, pressuresand pump failures, for example, as the LNG is being pumped from tanker 1(FIG. 1). Also, in the embodiment of FIG. 4, the speed loop vapor, i.e.,used to maintain a constant flow through the system, as discussed abovewith respect to the embodiment of FIG. 1, is taken from the dischargeport of the accumulator tank 118 in order to promote a more fully mixedsample.

Modifications to the embodiments of FIGS. 2 and 4 can been made to evenmore closely monitor the system pressure. For instance, during tankeroffloading, for example from tanker 1 (FIG. 1) into storage tanks 4, theBTU value calculations are affected by things, such as changes in tankerpump pressure and variations in storage tank filling levels.Specifically, events such as these change the speed loop flow ratewhich, in turn, can affect the value of the BTU calculation. Thus, bycarefully monitoring and controlling the flow rate in the speed loop,these types of anomalies are detected and accommodated.

It has also been recognized that when one or more of the tanker pumpssuddenly begin pumping, or otherwise change their pump rate, the BTUvalue reading is also affected in similar fashion to that mentionedabove. Accordingly, in accordance with a further embodiment, anadditional device can be added within the LNG cabinet to assist incontrolling the flow rate. For example, a flow controller (not shown),such as a Brooks 5850i Mass Flow Controller from Brooks Instrument ofHatfield, Pa., can be included within the LNG cabinet to control theflow rate within the speed loop. The location of the flow control devicewithin the speed loop is not critical. However, one viable location is,for example, on tubing 21 at the output of accumulator tank 18.

It would be understood for a person having ordinary skill in the artthat a device or method incorporating any of the additional oralternative details mentioned above would fall within the scope of thepresent invention as determined based upon the claims below and anyequivalents thereof.

Other aspects, objects and advantages of the present invention can beobtained from a study of the drawings, the disclosure and the appendedclaims.

1. A method of measuring the constituent components of liquid gas,comprising: receiving the liquid gas into a vaporizing device;selectively directing the received liquid gas into one or more of afirst and second stage vaporizer within the vaporizing device;converting the liquid gas directed to one or more of the first andsecond stage vaporizers into vapor gas; accumulating the vapor gas in astorage device; outputting the vapor gas accumulated in the storagedevice; directing the outputted vapor gas from the storage device to ameasuring device operable to determine the constituent components of thevapor gas.
 2. A method as claimed in claim 1, further comprisingcontrolling the pressure of the vapor gas directed to the measuringdevice to be less than or equal to 10 PSIG.
 3. A method as claimed inclaim 1, wherein the liquid gas received into the vaporizing device iseither liquid gas being off-loaded from a transportation device orliquid gas being recirculated from a static storage device.
 4. A methodas claimed in claim 1, further comprising continuously maintaining afresh sample of liquid gas flowing into the vaporizing device.
 5. Amethod as claimed in claim 1, further comprising determining a BTU valuefor the liquid gas corresponding to the vapor gas measured by themeasuring device and wherein the accuracy of the BTU value is betterthan ±5 BTU.
 6. A method as claimed in claim 5, wherein the accuracy ofthe BTU value is approximately ±1 BTU.
 7. A method as claimed in claim1, further comprising determining the composition of the vapor gas,wherein the individual components of the vapor gas are determined withan accuracy of within ±0.5 mole percent.